[1] Snow avalanches are a major natural hazard, endangering human life and infrastructure in mountainous areas throughout the world. In many countries with seasonally snow-covered mountains, avalanche-forecasting services reliably warn the public by issuing occurrence probabilities for a certain region. However, at present, a single avalanche event cannot be predicted in time and space. Much about the release process remains unknown, mainly because of the highly variable, layered character of the snowpack, a highly porous material that exists close to its melting point. The complex interaction between terrain, snowpack, and meteorological conditions leading to avalanche release is commonly described as avalanche formation. It is relevant to hazard mapping and essential to short-term forecasting, which involves weighting many contributory factors. Alternatively, the release process can be studied and modeled. This approach relies heavily on snow mechanics and snow properties, including texture. While the effect of meteorological conditions or changes on the deformational behavior of snow is known in qualitative or semiquantitative manner, the knowledge of the quantitative relation between snow texture and mechanical properties is limited, but promising developments are under way. Fracture mechanical models have been applied to explain the fracture propagation, and micromechanical models including the two competing processes (damage and sintering) have been applied to explain snow failure. There are knowledge gaps between the sequence of processes that lead to the release of the snow slab: snow deformation and failure, damage accumulation, fracture initiation, and fracture propagation. Simultaneously, the spatial variability that affects damage, fracture initiation, and fracture propagation has to be considered. This review focuses on dry snow slab avalanches and shows that dealing with a highly porous media close to its melting point and processes covering several orders of scale, from the size of a bond between snow grains to the size of a mountain slope, will continue to be very challenging.

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Snow avalanche formation

Snow studies have generally ignored the layered nature of snow covers. Having achieved a good understanding of the properties and processes in homogeneous snow, snow scientists should develop more insight into the evolution of the layers and their effects on overall snow response. Many of the outstanding problems in snow studies can only be solved by dealing with snow as a layered medium. The various mechanisms by which layers are generated, their effects, and some of the outstanding research problems are described here.

The layered character of snow covers

Snow stratigraphy studies at the fracture lines of dry slab avalanches often indicate a situation that consists of a harder thick layer overlying a thin weak layer. Prefracture stress conditions and progressive fracture of snow slabs are discussed here from the point of view that slow shear failure in the weak layer results in Griffith-like critical lengths there, resulting in rapid shear fracture and subsequent avalanche release. The important aspects of the timing of dry slab avalanche release and the relationship between the applied shear stress and the shear strength at the weak layer are discussed in this context. The results show that the main experimental facts on snow and field observations of snow slab avalanches are explainable from this approach, although direct, concrete proof is not presently available.

Shear fracture precipitated by strain softening as a mechanism of dry slab avalanche release

Review of dry snow slab avalanche release

Des tests de terrain ont ete developpes pour la mesure des limites de cisaillement et de tension pour de grands volumes de neige Des limites de cisaillement a la base ont ete mesures transversalement et longitudinalement pour des couvertures de neige, conduisant a des efforts hautement variables On analyse aussi les contraintes de traction dans la couche, des mesures de limite de rupture en traction en meme temps que des mesures de contraintes de pesanteur pour une plaque de neige, suggerent que les longueurs de deficit de quelques metres seulement sont suffisantes pour creer une rupture locale en traction Dans certains cas, cette rupture locale peut se propager a travers le reste de la pente (suivant la concentration des contraintes) et etre la cause d'une avalanche

Snow Stability Index

Existing theories of snow slab failure emphasize failure either in the tensile zone of the slab or along the slab base. The tensile zone theories do not explain measured fracture angles or the observed fact that a necessary condition for extensive fracturing is weakness in the basal plane. On the other hand, the basal theories do not explain where the tensile stress originates and why brittle failure commences in the tensile zone and propagates into the basal plane. A new model is proposed wherein the critical value of the maximum principal stress evaluated at the tensile zone is coupled to the inability of the slab to sustain basal stress. A scale analysis of the equilibrium equations shows that this model predicts the measured fracture angles and large values for the maximum principal stress.

E. R. LaChapelle

Papers

A theory of snow slab failure